DESCRIPTION (provide by applicant): In preparation for a global influenza pandemic, there is an urgent need for representative data from populations and settings where the pandemic is most likely to arise. There are no data on oseltamivir efficacy from Asian urban slum populations concerning duration of illness and viral shedding, nor whether efficacy depends on starting treatment <48 hours or >48 hours after illness onset. Finally, there are no data on the capacity of the drug, in such settings, to affect household and community transmission rates. This proposal aims to compare the duration of clinical illness among patients treated with oseltamivir vs placebo <48 hours and >48 hours after illness onset. It will compare the duration of viral shedding among all treatment groups vs placebo, risk of transmission to household contacts by treatment group and whether neuraminidase inhibitor use creates resistance. Secondarily it aims to measure the effect on influenza. A double-blind placebo controlled clinical trial design among a population in an urban slum under current influenza disease burden surveillance will be enrolled. Infection status will be confirmed by rRT-PCR. Patients >1 year old will be randomized to <48 hour and >48 hour treatment arms. Family members and neighbors will also be assessed by PCR and a basic reproductive number calculated (R0). These findings will address whether oseltamivir can affect illness duration and severity, affect transmission, incidence and resistance in high risk urban Asian settings where a pandemic is most likely to arise.